In mobile communications systems, mobile stations and base transceiver stations may set up connections through channels of a so-called radio interface. A certain frequency area is always allocated for use by the system. To have sufficient capacity in the mobile communications system on this limited frequency band, the channels which are in use must be used several times. For this reason, the coverage area of the system is divided into cells formed by the radio coverage areas of individual base transceiver stations, which is why the systems are often also called cellular radio systems.
FIG. 1 shows the main structural features of a known mobile communications system. The network comprises several inter-connected MSCs (Mobile Services Switching Centre). The mobile services switching centre MSC can set up connections with other mobile services switching centres MSC or with other telecommunication networks, e.g. ISDN (Integrated Services Digital Network), PSTN (Public Switched Telephone Network), Internet, PDN (Packet Data Network), ATM (Asynchronous Transfer Mode) or GPRS (General Packet Radio Service). Several base station controllers BSC are connected to the mobile services switching centre MSC. Base transceiver stations BTS are connected to each base station controller. The base transceiver station may set up connections with mobile stations MS. A network management system NMS may be used for collecting information from the network and for changing the programming of network elements.
The air interface between base transceiver stations and mobile stations can be divided into channels in several different ways. Known methods are at least TDM (Time Division Multiplexing), FDM (Frequency Division Multiplexing) and CDM (Code Division Multiplexing). The band available in a TDM system is divided into successive time slots. A certain number of successive time slots forms a periodically repeating time frame. The channel is defined by the time slot used in the time frame. In FDM systems, the channel is defined by the used frequency, while in CDM systems it is defined by the used frequency hopping pattern or hash code. Combinations of the division methods mentioned above can also be used.
FIG. 2 shows an example of a known FDM/TDM division. In the figure, frequency is on the vertical axis while time is on the horizontal axis. The available frequency spectrum is divided into six frequencies F1-F6. In addition, the frequency channel formed by each frequency is divided into repeating time frames formed by 16 successive time slots. The channel is always defined by the couple (F, TS) of frequency F and time slot TS used in the time frame.
In order to maximize capacity, channels must be reused in cells which are as close to one another as possible. Reuse of channels is limited by the interference caused to one another by the connections in the network.
FIG. 3 shows the emergence of interference caused to each other by simultaneous connections. In the figure three mobile stations MS1, MS2 and MS3 communicate with base transceiver stations BTS1, BTS2 and BTS3. The signal received by base transceiver station BTS1 contains a signal S1, which is sent by mobile station MS1 and which is showed by a solid line and the power of which depends on the transmission power used by mobile station MS1 and on fades on the radio path between mobile station MS1 and base transceiver station BTS1. Typically, the radio path fading is smaller with a shorter distance between base transceiver station and mobile station. In addition to signal S1, the signal received by the base transceiver station contains signal components I21 and I31 caused by signals sent by mobile stations MS2 and MS3. Components I21 and I31 will cause interference in the reception, if they are not filtered away from the signal received by the base transceiver station. Correspondingly, the signal sent by mobile station MS1 causes signal components I12 and I13 in the signals received by base transceiver stations BTS2 and BTS3 and these signal components may cause interference in the receptions. Components of a similar kind also emerge in the signals received by mobile stations from base transceiver stations.
If signal components I21 and I31 are on the same channel as signal S1, they can not be removed by filtering. Interference may also be caused by signals occurring on other channels than on the same channel. E.g. in systems using FDM frequency division, channels which are adjacent to one another on the frequency level are always slightly overlapping in order to use the frequency spectrum as effectively as possible, which will result in reception interference also from signals which are on the adjacent channel. Correspondingly, when using code division CDM, connections using codes that are too much alike will cause interference to one another. However, so-called neighbor channel interference caused by signals on other channels is considerably smaller than the interference caused by equally powerful signals on the same channel. The interference may also be affected e.g. by using frequency or time slot hopping. In frequency hopping, the frequency used by the connection is frequently changed, whereby the interference caused to one another by connections will be averaged. In time slot hopping again the time slot used in the connection is frequently changed. When using frequency or time slot hopping, the individual connection will not suffer an interference which is considerably worse than for others, but all connections will suffer interference of the same level.
The magnitude of interference caused by connections to each other thus depends on the channels used by the connections, on the geographical location of connections and on the transmission power used. These may be influenced through a systematic allocation of channels to different cells and through transmission power control taking the interference into account.
It is an objective in channel allocation to allocate such channels to the desired connections which may all be used at the same time while the signal quality remains acceptable. The invention to be presented in this application relates to but is not limited to fixed channel allocation FCA, wherein the required number of channels is allocated in advance to each cell with the aid of so-called frequency planning. In frequency planning it is ensured that the connections operating on channels allocated in different cells will not interfere excessively with each other. For interference control, the base transceiver station in each cell is given a maximum limit for the allowed transmission power. The distance at which one and the same channel can be reused so that the CIR (C/I, Carrier to Interference Ratio) remains acceptable, is called the interference distance while the distance at which one and the same channel is reused is called the reuse distance.
The same frequencies are reused according to a so-called reuse pattern. When using a channel structure with FDM/TDM division, typical reuse pattern sizes are 7, 9 and 12 cells, in other words, such patterns where the same frequencies are reused in every ninth or in every 12.sup.th cell. FIG. 4 shows an example of a reuse pattern the size of which is 9 cells. In FIG. 4 the frequencies are divided into 9 classes 1-9. One frequency class shown beside the cell in the figure is allocated for use by each cell. Only those channels may be used in the cell which belong to the frequency class allocated for use by the cell.
The reuse pattern can be made denser e.g. by using directed antennas or by reducing the demand made on the CIR ratio of the FDM/TDM signal. The carrier to interference ratio CIR demanded of the network can be lowered e.g. by improving the spectrum characteristics of the signal by using frequency hopping or a hash code of the CDM type or by using a more effective channel coding.
Other channel allocation methods besides FCA are at least DCA (Dynamic Channel Allocation) and HCA (Hybrid Channel Allocation) which is obtained as a combination of FCA and DCA. The different methods are described very thoroughly in the publication I. Katzela and M. Naghshineh: "Channel Assignment Schemes for Cellular Mobile Telecommunication Systems: A Comprehensive Survey", IEEE Personal Communications, pp. 10-31,June 1996.
Traffic is divided in such a way in the cell that some mobile stations are near the base transceiver station. Signals in the connections between these mobile stations and the base transceiver station typically experience considerably less fades than signals transmitted between the base transceiver station and mobile stations which are far from the base transceiver station. Channels used for connections between these mobile stations and the base transceiver station could be better reused than channels used by mobile stations located far from the base transceiver station and suffering greater fades in their connections. In fact, several overlapping reuse patterns can be used in a cellular radio network. Such a channel allocation method is called RUP (ReUse Partitioning). The channels are hereby divided into channel pools corresponding to different reuse patterns. Connections with low attenuation (typically connections between the base transceiver station and mobile stations located near it) which thus tolerate a higher interference level and cause less interference to other connections are directed to use a dense reuse pattern.
Using a carrier to interference ratio CIR which is higher than necessary will hardly improve the connection quality in digital systems but will just unnecessarily increase the interference caused to other connections. It is therefore sensible to control dynamically the transmission power used by the connections. A dynamic control of the transmission power aims at maintaining an adequate connection quality, however, at the same time minimizing the transmission power used. Besides the capacity advantages obtainable by minimizing the interference level, a considerable cut in the mobile station's power consumption is achieved through transmission power control.
The required power depends on fades on the channel between mobile station and base transceiver station, on interference caused by other connections and on ambient noise. On the one hand, by increasing the transmission power of the first connection it is possible to improve the carrier to interference ratio CIR of the connection, but on the other hand, extra interference will then be caused to other connections located nearby. The quality of other connections will hereby suffer. In response to increased interference and impaired quality, the other connections will raise their own transmission power, which will cause additional interference to the first connection. The situation is illustrated in FIG. 5.
The power control of three mutually interfering connections is examined in FIG. 5. The signal transmission power P1 of connection 1 is adjusted by the power controller PC1 so that the quality of the signal which is detected by the recipient and which mainly depends on the carrier to interference ratio CIR remains acceptable. The signal power C1 detected by the recipient of the signal depends on transmission power P1 and on the attenuation caused to the signal by the radio channel between sender and recipient. The attenuation is typically reduced with a shorter distance between mobile station and base transceiver station. Interference is caused to the received signal by ambient noise N1 and by interference l1 caused by the transmitters of other connections. Interference l1 depends on attenuation on the radio path between interfering transmitters and the recipient and on transmission powers P2 and P3 of the transmitters. The total signal S1 detected by the receiver is formed of the sum of signal C1, interference I1 and noise N1. If the information sent in signal C1 can not be adequately reconstructed from the received signal S1, then power controller PC1 will raise the signal transmission power P1. Correspondingly, if power P1 can be decreased so that the sent information can still be adequately reconstructed, then the power controller will reduce the transmission power P1. Power controllers PC2 and PC3 operate on the same principle. Only those mobile stations which experience little fading on the radio path and which are typically located near their base transceiver station need less transmission power and they cause less interference to others than do those mobile stations which experience more fades and which are typically located far away from the base transceiver station.
Interference can be reduced not only through systematic channel allocation and power control, but also by using directed antennas, whereby the same signal level can be achieved in the receiver with a lower transmission power.
The connection quality is affected not only by the carrier to interference ratio CIR indicating the quality of the radio channel, but also by the sensitivity to errors generated at radio channel of the information signal transmitted on the channel. The information can be made better to tolerate transfer errors by processing it with channel coding and interleaving before sending it to the transmission channel and by using retransmission of faulty data frames.
The purpose of channel coding is both to make the information transfer better tolerate transfer errors and to detect transfer errors. In channel coding such redundancy is added before transmission to the user data which can be used at the signal reception end for remedying errors caused by the radio channel and for detecting such errors which can not be remedied. Channel coding improves the interference tolerance, but on the other hand, it increases the band width necessary for information transfer.
Bit errors occurring on the radio path are typically error bursts having a length of several bit cycles. It is always easier to correct individual bit errors than a series of several successive faulty bits. The probability of occurrence of several successive faulty bits can be significantly reduced through bit interleaving, where the order of bits is mixed in a predetermined manner before sending the signal on the radio path. When the order between bits is restored to the original order at the reception end, those bits where errors have been caused by a burst-like interference on the radio path are no longer adjacent to one another, whereby the errors can be detected and corrected more easily. Interleaving makes more effective the correcting and detection of errors, but on the other hand it will cause some additional delay in the data transfer.
In digital mobile communications systems, information is always sent in frame shape, and if a data frame is found faulty, it can be retransmitted in systems supporting retransmission. By using more powerful channel coding and by retransmission it is possible to forward user data to the recipient with sufficient faultlessness even over a poorer radio channel. The use of retransmission will of course add to the delay in information transfer.
As the number of mobile station subscribers is increasing and those applications, such as multimedia applications, which demand a large band width become more general, state-of-the-art methods of channel allocation are no longer effective enough. It is an objective of the present invention to alleviate this problem by making channel allocation even more effective. This objective is achieved with the method described in the independent claims.